From TARDIS Index File, the free Doctor Who reference.

The Missing Adventures series of novels by Virgin Books, was launched in 1994 following the success of its New Adventures line. Much as the NA series featured Seventh Doctor adventures with more detail and mature content than previously seen on TV and in the Target Books novelisations, the MA series focused on the first six Doctors. There were occasional multi-Doctor stories and crossovers with the NA line. One book, Downtime, didn't feature the Doctor but was an adaptation of an independent film production featuring several former companions. Another MA release, The Ghosts of N-Space, was an adaptation of a Third Doctor radio play.

One release, Who Killed Kennedy, is generally considered a Missing Adventures release as it features several past Doctors, but it was in fact published as a standalone work.

The Missing Adventures line was discontinued in 1997 after BBC Books took back the franchise licence from Virgin (it is speculated that had Virgin kept the licence it would have begun publishing Seventh Doctor adventures under the MA line, and indeed that incarnation does appear in one post-TV movie release, Cold Fusion); BBC subsequently launched its own version, the BBC Past Doctor Adventures line.

Related to the MA line, but considered separate, was the Virgin Decalogs series of books.

Notes

The first Missing Adventure Goth Opera was a sequel (and also prequel, from the Doctor's point of view) to the New Adventure novel Blood Harvest. This was an obvious ploy to get readers of the New Adventures books to buy both books.

Downtime novelised the direct-to-video story of the same name. It did not featured the Doctor prominently but did feature a cameo by one of his future incarnations.

Who Killed Kennedy, although it focuses on the Third Doctor, was published as a standalone novel, not a Missing Adventure, but is listed here for the sake of convenience. Co-credited with its fictional protagonist, James Stevens, the book took the form of a mock "nonfiction exposé" of UNIT and its activities. It re-visited the events of the Third Doctor's time with UNIT from Stevens' perspective and gave an Earth-bound view of events, with the Doctor put in short appearances in various incarnations. It came out in the same month as The Eye of the Giant.